1. Field of the Invention
This invention relates to modulation circuits, and in particular, to a circuit providing high speed modulation of a semiconductor diode laser or light emitting diode.
2. Description of the Prior Art
It is commonly known that the amplitude (intensity) of a laser beam emitted from a semiconductor diode laser can be modulated by modulating the injection current supplied to the diode laser. Thus, when a diode laser is used as a source of light in optical communications, the laser beam, which is the carrier, can be directly modulated by the injection current.
The modulation is typically supplied by external circuitry. Depending on a number of factors, the circuitry may derive from integrated logic circuits such as TTL (Transistor-Transistor Logic) and ECL (Emitter Coupled Logic). However, digital modulation with standard IC logic families is often difficult due to:
(1) lack of sufficient modulation current,
(2) inappropriate logic voltage levels, and
(3) incompatibility of the IC with a grounded p-side (anode) connection of the laser.
As is well-known, the laser's heat-producing p-side is usually directly bonded to a heat sink package. Since good circuit practice requires grounding large metallic components it is therefore desirable to ground the heat sink.
An article in Electronic Design, July 19, 1974, pages 96-99, discloses use of an ECL gate to modulate light emitting diodes (LED) at high data rates. Extension of the concept to drive double heterostructure AlGaAs diode injection lasers is also disclosed. However, the anode is connected to V.sub.bias, which is incompatible with the grounded anode for good heat sinking as previously mentioned.
U.S. Pat. No. 4,009,385, which describes a pre-bias control circuit, includes a typical emitter coupled transistor driving circuit for the laser. However, the transistor pair disclosed requires an external reference voltage, which may not be available or compatible with standard logic IC's. Further, the modulation current can change with temperature or with variations of input signal (due to loss or component variation).
A need remains for a modulator for a diode laser compatible with integrated circuit logic families.